In recent years, application of a surface light source has spread and thus the surface light source has been widely used not only in the field of an advertisement display device and a display device for a personal computer, but also in the field of a liquid crystal display television or the like.
In addition, miniaturization and high efficiency are required for inverter circuits for driving those surface light sources.
Here, a relationship between the recent changes of the inverter circuit for a cold cathode fluorescent lamp and the invention of Japanese Patent No. 2733817 is stated as follows.
As for the inverter circuit for a cold cathode fluorescent lamp, a collector resonance type circuit (refer to FIG. 17) has been widely used. This collector resonance type circuit is referred as another name to as “a Royer circuit” in some cases. However, the proper definition of the Royer circuit is such that the inversion of a switching operation is performed in a state in which a transformer is saturated. Thus, the inverter circuit which performs the inversion operation by utilizing the resonance on the collector side is desirably referred to as “a collector resonance type circuit” or “a collector resonance type Royer circuit” in distinction from the Royer circuit.
Now, the initial inverter circuit for a cold cathode fluorescent lamp did not utilize the resonance method of a secondary side circuit at all, and the so-called closed magnetic circuit type transformer having a small leakage inductance was used in a step-up transformer. In the background of the times, the so-called closed magnetic circuit type transformer method a transformer having a small leakage inductance in terms of recognition of a person skilled in the art. In addition, the leakage inductance of the step-up transformer in the inverter circuit was recognized such that it reduced an output voltage on a secondary side of a transformer and was not preferable, and thus was desirably as small as possible.
As a result, a resonance frequency of the secondary side circuit of the transformer in the background of the times was judged to have no connection with an operating frequency of the inverter circuit. Thus, the resonance frequency of the secondary side circuit used to be set to a much higher frequency than the operating frequency of the inverter circuit so as to exert no influence on the operating frequency of the inverter circuit. In addition, a ballast capacitor Cb is essential for stabilization of a lamp current.
Next, with respect to the inverter circuit for a cold cathode fluorescent lamp, an inverter circuit shown in FIG. 18 is known. However, this inverter circuit is one disclosed in Japanese Unexamined Patent Publication No. Hei 7-211472, and has come into wide use as the co-called three-time resonance circuit in which as shown in FIG. 19, the resonance frequency of the secondary side circuit is three times as high as an oscillation frequency of a primary side circuit. A step-up transformer in which a leakage inductance value is increased to some degree is suitable for one used in this case.
In this case, as shown in explanatory diagrams of FIGS. 20A to 20D, the signal having the oscillation frequency of the inverter circuit and the third-order harmonics are composed with each other to generate a signal having a trapezoid waveform.
Then, the current which is actually caused to flow through the cold cathode fluorescent lamp of the three-time resonance circuit shows a waveform as shown in FIG. 21.
There is confusion in the name of the step-up transformer in this case. There is controversy as to whether or not the step-up transformer may be referred to as “the so-called closed magnetic circuit type transformer” which is said among those skilled in the art. Thus, the definition of the name of the step-up transformer becomes vague. There is a problem as to how a state is described in which the leakage of the magnetic flux is much though the magnetic path structure is closed. A problem still exists such that those terms are not the special technical term each in which the state as described above is supposed.
The shape of the transformer which is actually used in the so-called three-time resonance is flat as shown in FIGS. 22A and 22B. Thus, though the magnetic path structure is closed, the leakage of the magnetic flux is considerably more than that of the conventional one. That is, that transformer has a large leakage inductance value.
In any case, this technical idea (refer to FIG. 18) is such that the leakage inductance value of the step-up transformer is increased to some degree, whereby a resonance circuit is structured by using a leakage inductance (Le in FIG. 18) and a capacitance component obtained on the secondary side of the step-up transformer, and a resonance frequency of the resonance circuit is set to a frequency three times as high as the operating frequency of the inverter circuit in order to generate a third-order harmonics in the secondary side circuit (refer to FIG. 19), thereby obtaining a lamp current waveform having a trapezoid shape (refer to FIG. 20D). A ballast capacitor C2 in this case, though being a ballast capacitor, functions as a part of a resonance capacitor.
As a result, as disclosed in the invention of Japanese Unexamined Patent Publication No. Hei 7-211472, the conversion efficiency of the inverter circuit is considerably improved, and also the step-up transformer is further miniaturized. In addition, this technical idea about the three-time resonance has become the basis of the collector resonance type inverter for a cold cathode fluorescent lamp from recent years up to the present time. Thus, it is not too much to say that the technique concerned is utilized in the great majority of a considerable number of collector resonance type inverter circuits which currently come into wide use.
Next, the invention of Japanese Patent No. 2733817 that becomes the basis of the present invention was disclosed, whereby more dramatic miniaturization and high efficiency promotion of the step-up transformer have been realized. The present invention began to be widely implemented in about 1996, and thus has greatly contributed to the miniaturization and high efficiency promotion of the inverter circuit used in a note type personal computer. The invention concerned is the invention such that the operating frequency of the inverter circuit and the resonance frequency of the secondary side circuit are made nearly agree with each other. The leakage inductance value of the step-up transformer in the three-time resonance is further increased, and at the same time the capacitance component of the secondary side circuit is increased, thereby realizing the invention concerned.
This technique utilizes such an effect that when the inverter circuit operates in the vicinity of the resonance frequency of the secondary side circuit, an exciting current which is caused to flow through a primary winding of the step-up transformer becomes less. Thus, a power factor when viewed from the primary winding side is enhanced, and a copper loss of the step-up transformer decreases.
Also, after the disclosure of the invention concerned, a large number of driving method, which will be described later, such as separately excited driving method having a fixed frequency, and a zero current switching type driving method for detecting a zero current of the primary side winding to perform the switching have been used as the driving method of the primary side circuit in addition to the conventional collector resonance type circuit. A series of those peripheral techniques are related to the dependent inventions of the invention concerned, and contribute to the spread of the resonance technique of the secondary side circuit in the invention concerned.
When changes of the background technique relating to a series of those inverter circuits for the cold cathode fluorescent lamps are viewed from a viewpoint of the leakage inductance value of the step-up transformer, those changes can be regarded as the history in which as the generation of the inverter circuit has been renewed, the leakage inductance value of the step-up transformer has also increased and at the same time the resonance frequency of the secondary side circuit has been lowered.
The high efficiency promotion and miniaturization of the inverter circuit are realized by improving the step-up transformer and by suitably selecting the driving frequency of the step-up transformer. For this point, the inventor of the present invention discloses in detail the technique for the high efficiency promotion when viewed from the driving method side together with a graphical representation of FIG. 23 in the invention of Japanese Unexamined Patent Publication No. 2003-168585. FIG. 23 is a graphical representation explaining the technique for improving the power factor when viewed from the driving method side. In the diagram, an axis of abscissa represents a frequency, and a represents a phase difference between a voltage phase and a current phase in a primary winding of a step-up transformer. FIG. 23 explains that the power factor is improved as è becomes nearer zero.
On the other hand, as disclosed in U.S. Pat. No. 6,114,814-B1 and Japanese Unexamined Patent Publication No. Sho 59-032370, the technical idea asserting that the high-efficiency inverter circuit is provided by the zero current switching method is firmly advocated among those skilled in the art.
However, those technical ideas lack a viewpoint of the effect of the power factor improvement in the step-up transistor, and thus are not proper in that they assert that the high efficiency results from the reduction in exothermic quantity of switching transistor.
This point will hereinafter be described in detail.
The zero current switching method is one of power controlling method of the inverter circuit, and examples of the zero current switching type circuit as shown in FIGS. 24 and 30 are disclosed as typical ones in U.S. Pat. No. 6,114,814-B1 and Japanese Unexamined Patent Publication No. Sho 59-032370. In addition, the inventor of the present invention also discloses as the same technique as that described above in Japanese Unexamined Patent Publication No. Hei 8-288080 (refer to FIG. 29). This technique will be described based on U.S. Pat. No. 6,114,814-B1 as follows.
Here, FIGS. 25A to D correspond to FIGS. 11A to 11D shown in U.S. Pat. No. 6,114,814-B1, respectively, and FIGS. 26E to F and AA and BB correspond to FIGS. 11E and 11F, and FIGS. 12A and 12B shown in U.S. Pat. No. 6,114,814-B1, respectively.
In U.S. Pat. No. 6,114,814-B1, FIGS. 11A to 11D and FIGS. 12A and 12B show waveform diagrams explaining an operation of the zero current switching type circuit of the invention concerned. FIGS. 11A and 11B in U.S. Pat. No. 6,114,814-B1 show a state in which no power control is performed, FIGS. 11C and 11D in U.S. Pat. No. 6,114,814-B1 show a state in which the power control is performed, and FIGS. 11E and 11F in U.S. Pat. No. 6,114,814-B1 show a case where the zero current switching operation is intended to be performed in a state in which a phase of an effective voltage value leads a phase of an effective current value. In addition, diagrams as shown in FIG. 26 are shown in FIGS. 12A and 12B in U.S. Pat. No. 6,114,814-B1, and such FIGS. 12A and 12B show an example of control in a case of no zero current switching operation.
In those diagrams, FIG. 11A in U.S. Pat. No. 6,114,814-B1 shows a voltage applied to a primary winding of a transformer when a driving power is the maximum, and FIG. 11B in U.S. Pat. No. 6,114,814-B1 shows a current which is caused to flow through the primary winding of the transformer in that case. The zero current switching method in the inverter circuit for a cold cathode fluorescent lamp serves to detect timing at which the current value becomes zero to turn ON a switching element. When the driving power is the maximum, i.e., when a flow angle is set to 100% and thus no power control is performed, there is necessarily no phase difference between the phase of the effective voltage value and the phase of the effective current value which is given to the primary winding. That is, this method that the power factor is satisfactory.
Next, FIG. 11C in U.S. Pat. No. 6,114,814-B1 shows a voltage applied to the primary winding of the transformer when the flow angle is made small in order to control the driving power. Also, FIG. 11D in U.S. Pat. No. 6,114,814-B1 shows a current which is caused to flow through the primary winding in this case. In this diagram, a switching transistor is turned ON at timing at which the current value becomes zero. However, turn-OFF of the switching transistor is not caused at the timing at which the current value becomes zero. In this case, there is a phase difference between the phase of the effective value of the voltage applied to the primary winding and the phase of the effective value of the current caused to flow through the primary winding. As a result, the power factor in this case is not satisfactory.
On the other hand, while FIG. 26AA shows a case where the power control is performed with the flow angle being similarly limited, the control is performed such that the phase of the effective value of the voltage in the primary winding and the phase of the effective value of the current caused to flow through the primary winding become equal to each other under a condition in which the zero current switching method is disregarded. In this case, the power factor when viewed from the primary winding side of the transformer is really satisfactory, and thus the exothermic quantity of step-up transformer is less. However, this technique is not related to zero current switching method.
Here, the zero current switching method is in consistent with the technical idea for providing the high efficiency for the inverter circuit. In the technical idea of U.S. Pat. No. 6,114,814-B1, the zero current switching method is excluded in the states as shown in FIGS. 12A and 12B in U.S. Pat. No. 6,114,814-B1 because of the unsatisfactory conversion efficiency of the inverter circuit.
However, the comparative experiments made by the inventor show that the conversion efficiency of the inverter circuit in a case of the control method shown in FIGS. 26AA and BB is obviously higher than that of the inverter circuit in a case of the control method shown in FIGS. 25C and D.
In conclusion, it is false that the zero current switching method provides the high efficiency for the inverter circuit. The background causing such misunderstanding is as follows.
Especially, only when no power control is performed in the zero current switching method, the phase difference between the phase of the voltage applied to the primary winding of the step-up transformer and the phase of the current caused to flow through the primary winding thereof necessarily disappears. For this reason, the power factor of the step-up transformer is improved, the current caused to flow through the primary winding is reduced, and the current caused to flow through the switching transistor becomes the minimum. As a result, the exothermic quantity of primary winding of the step-up transformer and the exothermic quantity of switching transistor are reduced, so that the efficiency of the inverter circuit is improved. It seems that this fact is misidentified as that the high efficiency is provided by the zero current switching method.
The state as shown in 11A and 11B in U.S. Pat. No. 6,114,814-B1 corresponds to a case where no power control is performed. An operating state in this case becomes equivalent to an operating state of the general current resonance type inverter circuit. That is, it is found out that the high-efficiency inverter circuit is not provided by the zero current switching method, but really provided by the conventional current resonance type method.
The current resonance type inverter circuit is known as one for a cold cathode fluorescent lamp, and for example, a circuit as shown in FIG. 27 is generally used as the current resonance type inverter circuit. Such a current resonance type circuit has no dimming method in a case where only a structure of a basic circuit is adopted. Then, when the dimming is performed in the current resonance type inverter circuit, the dimming is performed by using a DC-DC converter circuit is provided in a preceding state of the current resonance type inverter circuit in order to perform the dimming.
FIG. 28 shows an example of a dimming circuit in an inverter circuit for a cold cathode fluorescent lamp in which the conventional current resonance type circuit is combined with a DC-DC converter circuit provided in a preceding stage thereof. In this example, switching method Qs, a choke coil Lc, a fly-wheel diode Ds, and smoothing capacitor Cv constitute the DC-DC converter circuit.
On the other hand, a technique for performing the dimming by using an improved current resonance type circuit. FIG. 29 shows a dimming circuit which the inventor of the present invention disclosed in JAPANESE UNEXAMINED PATENT PUBLICATION 8-288080 A. In this dimming circuit, timer circuits 10 and 11 detect a zero current, and after a lapse of a predetermined period of time, a frequency controlling circuit 12 turns OFF switching elements 2 and 3. Each of the timer circuits 10 and 11 is structured in the form of an RS flip-flop, and set with the zero current and reset after a lapse of a given period of time. This dimming circuit is one for performing the dimming by utilizing a method of turning OFF switching method after a lapse of a given period of time after detecting the zero current to turn ON the switching method.
The same technique as that described above is disclosed in FIG. 9 as well of U.S. Pat. No. 6,114,814-Bi, FIG. 30 is a circuit diagram shown in FIG. 9 of U.S. Pat. No. 6,114,814-B1. In this circuit, an RS flip-flop 172 is set by a zero current, and reset after a lapse of a given period of time. Each of the techniques disclosed in U.S. Pat. No. 6,114,814-B1 and Japanese Unexamined Patent Publication No. Hei 8-288080 is such that at the same time that the zero current is detected to turn ON the switching method, the RS flip-flop is set, and reset after a lapse of a given period of time, thereby turning OFF the switching method. Each of these techniques gives the switching method of the current resonance type circuit the dimming function, and thus has such a feature that a phase of the current laps a phase of the effective voltage during the dimming phase. Thus, those techniques are established based on the completely same technical idea, and nearly identical in realization method to each other.
The inventor himself verifies that if the dimming is performed by utilizing the technique of Japanese Unexamined Patent Publication No. Hei 8-288080, when the cold cathode fluorescent lamp or hot cathode lamp is controlled so as to become considerably dark, a much current is caused to flow through the transistor constituting the switching method and thus the transistor is heated.
[Patent document 1] Japanese Patent No. 2733817
[Patent document 2] Japanese Unexamined Patent Publication No. Sho 59-032370
[Patent document 3] Japanese Unexamined Patent Publication No. Hei 7-211472
[Patent document 4] Japanese Unexamined Patent Publication 15 No. Hei 8-288080
[Patent document 5] Japanese Unexamined Patent Publication No. 2003-168585
[Patent document 6] U.S. Pat. No. 5,495,405
[Patent document 7] U.S. Pat. No. 6,114,814-B1
In the power controlling method of the inverter circuit using the conventional collector resonance type circuit, as shown in FIG. 17, the dimming of the discharge lamp is generally controlled by the DC-DC converter circuit provided in the preceding stage of the collector resonance type circuit.
In addition, the operating frequency of such a DC-DC converter circuit generally has no connection with the oscillation frequency of the inverter circuit. Thus, the switching timing depends on neither the zero voltage nor the zero current. In spite of this situation, an exothermic quantity of switching method of the DC-DC converter circuit is not so much. Thus, the DC-DC converter circuit does not reduce the conversion efficiency of the overall inverter circuit.
The reason that the conversion efficiency is low in the conventional inverter circuit is that the conversion efficiency of 35 the collector resonance type circuit is low, and is not that the conversion efficiency of the DC-DC converter circuit is low. This method that the zero current switching method does not necessarily contribute to the improvement in the conversion efficiency of the inverter circuit.
In order to verify this fact, the experiments were made in which as shown in FIG. 28, the collector resonance type circuit in the conventional inverter circuit was replaced with the current resonance type circuit. As a result, it was confirmed that though there arose such a problem that when the power source voltage was low, the satisfactory results could not be obtained in the conventional half-bridge type current resonance type circuit because the utilization efficiency of the power source was low, when the power source voltage was high, the conversion efficiency of the inverter circuit was dramatically increased.
Here, a relationship between the current resonance type circuit and the zero current switching method is summarized as follows.
When no flow angle is limited in the zero current switching method and thus no power control is performed, since as shown in FIGS. 25A and B, the phase difference between the phase of the effective voltage value and the phase of the current when viewed from the primary winding side of the transformer is small and thus the power factor is satisfactory, the conversion efficiency of the inverter circuit is also satisfactory.
Next, when the power control is performed by using the zero current switching method, the waveform of FIG. 25C is adopted in order to perform the power control. In this case, when the power control is performed with the flow angle being limited, as shown in FIGS. 25C and D, the phase difference between the phase of the effective voltage value and the phase of the current becomes large, the lowering of power factor will increase the current, and the copper loss increases accordingly, so that the thermal loss of primary winding of the transformer increases. In addition, the exothermic quantity of transistor constituting the switching method increases since the current increases. As a result, the conversion efficiency of the inverter circuit is reduced.
That is, the factor which most contributes to the improvement in the conversion efficiency of the inverter circuit for a cold cathode fluorescent lamp is not the zero current switching method. However, the effect of improving the power factor of the step-up transformer under the specific condition which is provided by the zero current switching method predominantly contributes to the improvement in the conversion efficiency of the inverter circuit for a cold cathode fluorescent lamp. The case under the specific condition corresponds to a case where no flow angle is limited. This is fit for the current resonance type circuit.
This is checked in detail as follows.
FIG. 31 is a graphical representation in the form of which the relationship between the voltage shown in FIG. 25C and the current shown in FIG. 25D is summarized and which explains a relationship between the voltage and current in the primary winding of the transformer in the zero current switching method, and their phases. That is, FIG. 31 is a graph in a case where the flow angle in FIGS. 25C and D when the power control is performed is set to about 25%. In this case, a point a in FIG. 31 represents timing at which the switching method is turned ON, while a point b represents timing at which the switching method is turned OFF. In addition, a waveform Es is one of the voltage applied to the primary winding of the transformer, a waveform Er is one of the effective value of the voltage in the primary winding of the transformer, and a waveform Iw is one of the current caused to flow through the primary winding of the transformer. As can be seen from this diagram, firstly, the turn-ON of the switching method corresponds to the zero current timing, while the turn-OFF thereof does not correspond to the zero current timing. In addition, when the zero current switching control is performed in such a manner, the waveform (current) Iw necessarily lags the waveform (effective voltage value) Er.
This is checked in more detail as follows.
Checking the relationship between the delay angle and the flow angle (duty ratio) with respect to how the waveform (current) Iw lags the waveform (effective voltage value) Er, a simple inverse proportion relationship is obtained between them. FIG. 32 shows this situation in the form of a graph.
FIG. 32 is a graphical representation which is obtained by calculating how the phase of the effective voltage value and the phase of the current change along with the change in flow angle. This diagram explains that when the flow angle is 25%, the lag angle of the current with respect to the voltage is 67.5 degrees. From this diagram, when the flow angle (duty ratio) is set to 25%, the phase lag of the current with respect to the voltage is obtained as about 67.5 degrees.
Next, FIGS. 33 and 34 are respectively a diagram and a graphical representation which are obtained by examining the power factor.
In FIG. 33, when a load current obtained through primary side conversion is assigned a, an exciting current is expressed in the form of tan °, and the current caused to flow through the primary winding is expressed in the form of 1/cosO (a reciprocal number of the power factor).
FIG. 34 is a graph which represents a relationship among a load current obtained through the primary-side conversion, the exciting current and the current caused to flow through the primary winding when the power factor is examined, and which explains that the much exciting current is caused to flow and a reactive current increases as the lag angle becomes larger.
In FIG. 34, a composite current ratio method 1/cos 6 (the reciprocal number of the power factor). FIG. 34 shows a relationship between a current lag angle 0 as lag of the current phase with respect to the phase of the effective voltage value and 1/cos 0 (the reciprocal number of the power factor). What times the primary winding current as much as the load current is caused to flow is examined as shown in FIG. 34. The current caused to flow through the primary winding when the phase of the current lags the phase of the effective voltage value by 67.5 degrees is 2.61 times as much as that when the phase of the current does not lag the phase of the effective voltage value at all. For this reason, it is understood that the power factor is very poor, and the exothermic quantity of primary winding increases due to an increase in copper loss. In addition, it is understood that the exothermic quantity of transistor constituting the switching method also increases for the same reason.
That is, when the power is controlled by using the zero current switching method, performing the power control by using the flow angle controlling method disclosed in U.S. Pat. No. 6,114,814-B1, Japanese Unexamined Patent Publication No. Hei 8-288080 or Japanese Unexamined Patent Publication No. Sho 59-032370 is concluded from a viewpoint of improvement in the power factor as follows.
The excellent conversion efficiency is obtained in the inverter circuit in a state in which the flow angle is wide, i.e., in a state in which the lag of the phase of the current with respect to the phase of the effective voltage value is small. However, when the flow angle is small, the lag of the phase of the current with respect to the phase of the effective voltage value is large, and thus the power factor becomes poor. As a result, the current caused to flow through the primary winding of the transformer increases, whereby the conversion efficiency of the converter circuit becomes worse. In particular, when the flow angle is narrow, as the lag of the current phase approaches 90 degrees, the reactive current abruptly increases, and the conversion efficiency remarkably becomes worse.
In such a state, concretely speaking, in a case where an A.C. adapter is used when the zero current switching method is applied to the notebook type personal computer, the power source voltage becomes high. Under this condition, however, when the liquid crystal panel is desired to become dark by limiting the power, the lag of the current phase becomes the highest. This case is actually accompanied with the remarkable exothermic quantity of inverter circuit.
Moreover, when the current control is performed by the zero current switching method, there is encountered such a problem that 35 it is impossible to avoid the change in operating frequency of the inverter circuit.
Here, the evident fact is that the technical idea called the zero current switching is not necessarily essential to the structure of the inverter circuit having the excellent conversion efficiency in a state in which the power control is performed. Far from that, the technical idea called the zero current switching is rather harmful than is not necessarily essential thereto. In order to structure the inverter circuit having the excellent conversion efficiency, it is necessary to exclude the above-mentioned technical idea and adopt a method of making the power factor in the primary winding of the step-up transformer the best.
As for other method of implementing the technical significance of Japanese Patent No 2733817 (U.S. Pat. No. 5,495,405), the separately excited driving method having a fixed frequency is used in many cases. In such cases, when the resonance frequency of the secondary side circuit is shifted or the driving frequency of the primary side driving circuit is shifted due to the dispersion in circuit constants, the driving cannot be performed at the optimal resonance frequency at which the power factor improvement effect is actualized in some cases.
When the resonance frequency of the secondary side circuit and the driving frequency of the primary side circuit are shifted, this makes the power factor of the inverter circuit extremely worse. From this, when the separately excited driving method having the fixed frequency is used, a Q value of the resonance circuit of the secondary side circuit is lowered to obtain the broad resonance characteristics, thereby coping with the frequency shift. From such a reason, it is difficult to increase the Q value of the secondary side resonance circuit in the separately excited driving method having the fixed frequency.
In addition, when the zero current switching method and the separately excited driving method having the fixed frequency is used, it is possible to structure the inverter circuit having the high efficiency. In this case, however, there is encountered such a problem that there are a large number of constants of the circuit components, and the zero current switching method or the separately excited driving method is expensive. On the other hand, while the collector resonance type circuit involves such a problem that the efficiency is poor and there is the much calorification, it is inexpensive. From this, the collector resonance type circuit is still firmly supported as the method for reducing the cost. Those problems are an obstacle to the spread of the high-efficiency inverter circuit.